This invention relates to the production of shaped clay particles suitable as catalyst supports and catalysts employing the same as supports.
As is well known in the art of catalysis, the processes of heterogeneous catalysis require the presence of discrete particles through which the reacting products may be passed under suitable conditions to be converted as required. Depending on the nature of the process, the discrete particles may be positioned in a fixed bed, a moving bed, or suspended in the reactants as in the so-called fluid catalytic processes.
Many catalytic metals and compounds, in their catalytic state are required to be supported by such discrete particles since by themselves in bulk form they are either non-catalytic or so finely divided as not to be suitable for such uses.
The prior art practice is to employ discrete particles of relatively low catalytic properties as bodies on which the catalyst may be distributed. These materials are referred to as catalyst supports. It is an object of this invention to produce a support from a sub-bentonite clay. Such clays after acid treatment are being employed as adsorbents and as catalysts.
I have found that I may produce a useful catalyst support from the acid-treated sub-bentonite clays referred to above.
In so doing, I produce an increase in the SiO.sub.2 content into the range above about 80% to substantially 100% SiO.sub.2 content of the clay on a volatile free basis. In so doing, I impair and may substantially destroy the crystallinity of the sub-bentonite as measure by its X-ray diffraction pattern.
The effect of this treatment is to cause a redistribution of the pores in the various pore size ranges. The treatment to cause an SiO.sub.2 increase is carried out to the desired degree as to result in a substantial increase in the percent of the pores that are in the range above 30 A radius and a substantial decrease in the percent pores and may cause a complete destruction of the pores in the range below about 20 to 30 A. Additionally, the total pore volume per gram increases substantially as the SiO.sub.2 content is increased and thus also the total volume that is in the pores of greater than about 25 to 30 A radius.
The process results also in a substantial increase in the ratio of the total pore volume to surface ratio and an increase in percent of the pore volume is contributed by the pores of radii which are of greater than 30 A radius.
The redistribution of the pore volume so that the pore volume in pores greater than 30 A radius is increased provides more ready access to the reactants and permits a more ready escape of the products of the reaction.
Processes operating at high temperature as, for example, in petroleum cracking, reforming, hydroforming, and hydrodesulfurization, operate with feed stocks which result in coke deposition. Such feed stocks in many cases contain metals or metal compounds, for example, nickel, copper, platinum and other catalytic elements or compounds as for example, oxides and sulfides, and vanadium, which as such or as a result of reaction are converted into compounds which deposit in the pores of the catalyst. The pellets in such prior art processes contain pores which are in substantial fraction of the pore volume of less than 20 Angstroms. It has been suggested in order to increase the utility of the catalyst to increase the surface to volume ratio by limiting the radius of cylindrical pellet or to form it in a non-cylindrical shape. Substrates having large pores such as the alumina hydrates are when carrying deposited catalytic nickel compounds have the major portion of this pore volume in pores of radii of 20 to 45 A. See Gustafson, U.S. Pat. No. 3,966,644 and also E. Carlson, et al, U.S. Pat. No. 3,562,800. The fine pores, for example, those under 20-30 Angstroms pore radii, will clog more rapidly than pores of greater radius.
The increase in the total pore volume and of the percent of the volume contributed by the pores of more than 30 A radius results in an increased carrying capacity for catalytic elements or compounds for which the pellet act as a carrier either as a support or matrix. The process of my invention also increases the thermal stability so that the pellet retains its pore volume and surface area characteristics when heated to a much higher temperature than the prior art acid-treated sub-bentonite clay.
It is, therefore, an object of my invention to produce a catalyst support based on a sub-bentonite montmorillonite clay which has a major proportion of the pore volume in pores above about 20 A radius.
It is further an object of my invention to produce a catalyst support based on a sub-bentonite montmorillonite clay in which the total pore volume in pores of greater than 30 A radius is substantially greater than the pore volume in pores of 20-30 A radius or in pores of less than 20 A radius.
As has been described above, for many catalytic processes, either of the fixed-bed type or of the moving-bed type, the catalyst cannot be used in powder form as it would be blown out of the unit. It is, therefore, required that the catalyst support be in the form of a shaped particle, for example, a pilled or extruded particle, herein referred to as a pellet.
These may be formed when using acid-treated clays by limiting the acid activation of the clay to leave sufficient plasticity to permit them to be formed, for example, by extrusion or by spray drying. In such case, the available pore volumes are present in excessive proportions in a narrow range of pore dimensions, e.g., under about 20 Angstroms radius.
Characteristic pellet dimensions for extruded particles are of the order 1/20 to 1/4" in the minimum dimension for example of its cross section and about 1/16 to 1/4" of an inch in length. The microspheres formed by spray drying are in the range of 50-100 microns. These figures are intended as illustrations and not as a limitation. The pellet dimensions and shape are chosen to obtain the desired stability of the bed and retention of its flow characteristics. For this purpose, it is desirable that the pellets have the required structural strength to resist undue crushing and thus maintain their dimensional integrity.
Where the particles are in the nominal form of microspherical shape in the range of about 50 to 100 microns for use in fluid catalytic processes it is desirable that they have the hardness to resist the abrasion which they encounter in such processes.
A serious problem with shaped catalysts and catalyst supports either of the pellet or spray dried microsphere kind results from their limited structural strength. Movement of the particles in transportation, charging the catalytic equipment and motion in the equipment during processing, causes fragmentation of the particles and production of fines. In the moving bed process, these fines are removed with the exiting fluid from the bed and may unavoidable be discharged to the air. In the fixed bed process, the fines may lead to plugging the reactor, in which case the bed must be removed and the fines separated. Both difficulties are undesirable.
It is one of the advantageous properties of the particles of my invention that they have sufficient structural strength to avoid excessive fragmentation in processes described above.
Another useful property of the catalyst particles of my invention arises from the excellent heat stability of the catalyst. In many catalytic processes operating at high temperatures on carbon-containing compounds, the catalysts become contaminated by carbon or carbon-containing products of catalysis. It is conventional in such processes to regenerate the catalyst by burning the residue. The resulting temperatures are usually in excess of the catalysis temperatures.
It is one of the useful properties of my invention that the particles produced in the process of my invention have a surprising heat stability in that heating at temperatures in the range of 1500-1600.degree. does not materially depreciate the pore volume or surface areas of the catalyst support of my invention.
Where the particles are used as catalyst supports for catalytic compounds which are deposited in the pores of the catalysts, the pore volume, particularly in the range of 30 A radius and higher, is an important factor in the carrying power of the catalyst, that is, the weight of the catalytic compounds or metal which may be carried by a unit weight of the substrate. The greater the weight percent of the catalyst compounds or metal, the less the total weight or volume of the pellets required for the same weight of the catalyst in the reactor, that is, for any given catalyst to reactant ratio. This not only results in an economy in the reactor size but also in an improved result.
The particles of my invention are superior in the above properties as compared to conventional acid-treated sub-bentonite clays.
In order to form the acid-treated clay into particles, to act as catalyst supports, the original acid treatment with, for example, sulfuric, hydrochloric or nitric acid, must be limited so that the plasticity of the clay be not destroyed. In the conventional acid treatment, for example, of sub-bentonite (montmorillonite) clays, the SiO.sub.2 content is raised to about 70-75% by weight on a volatile free basis.
As described above, in order to obtain particles which have desirable pore volumes and favorable pore microsphere distribution, surface heat stability, and mechanical strength, the acid treatment is carried out according to my process to extract additional Al.sub.2 O.sub.3 from the montmorillonite lattice by treatment with one of the above acids. But if such degree of extraction is made on a powdered unextracted clay, the plasticity of the clay is impaired to a degree as to make the shaping of the acid-treated clay impractical.
I have, however, found that a calcined particle formed of plastic acid-treated sub-bentonite clay either by extrusion or by spray drying may be further acid treated with one of the above acids to increase its SiO.sub.2 content to above 80-85% by weight and on recalcination will produce a catalyst base with desirable pore size distribution where the pore volume in pores of 20-30 A radius and in pores above 30 A radius is made greater than in the pores below 20 A radius and is also of suitable hardness and good heat stability.
While I do not wish to be bound by any theory as to the effect of this improved pore size distribution, it may be pointed out that the opening up of the pores by increasing the pore radii and the volume of the pores increases the ready access of the reactants to the enlarged pores and to the remaining pores of smaller diameter due to the spatial interconnection of the pores. The plugging effect of metal compounds, and of carbon deposited during subsequent use as a catalyst or catalyst carrier is thus minimized. In addition, as described above, the carrying power for catalytic compounds and metals is materially improved.
I am thus able to obtain a pelleted clay or a spray dried microsphere base having a SiO.sub.2 content above 80% at which SiO.sub.2 content, the acid-treated sub-bentonites of this SiO.sub.2 content are not sufficiently plastic to be shaped into pellets.
The additional acid treatment removes additional lattice cations which may deleteriously affect the activity of catalytic metals or compounds which may be deposited on the catalyst support.
I have achieved a clay shaped, for example, into a pellet or micropshere of relative high surface area and pore volume in an extended range of pore radii, and of suitably high resistance to disruptive forces such as crushing or abrasion and good thermal stability by first activating a sub-bentonite clay by acid leaching to remove the exchange cations from the lattice and to partially remove the alumina from the clay lattice to a degree which does not destroy the plasticity of the clay, and I am thereby able to shape the clay to the desired shape. I set the structure by calcining it. At this point in the treatment, the volume in pores of less than 20 A radius is greater than in pores of more than 30 A radius. I then am able to further extract the alumina content of the clay by a further acid treatment and calcination without destroying the structural integrity of the shaped clay. The degree of leaching and calcination is sufficient to produce a nonswelling, nonplastic rigid particle of suitable hardness to resist the crushing of the shaped clay and to be stable at high temperatures in the processes of the catalysis referred to above.
The process causes a destruction of the pores of smaller radii and an increase in the pores of larger radii. The product is a clay particle with the pore volume in pores of radii greater than 30 A substantially greater than in the pores of less than 20 A radii, but it is nevertheless mechanically and thermally stable.
In the preferred embodiment of my invention, I accomplish this result by regulating the temperature at which the initial calcination process is carried out. The degree of extraction after initial calcination may be up to about 100% extraction of alumina to produce a pellet containing up to 100% of silica without impairing the structural integrity of the pellet.
In order to obtain a particle of suitable hardness, i.e., mechanical stability, particularly when employing a sub-bentonite type of clay which has been extracted by conventional treatment to above 70-75% SiO.sub.2, the temperature of calcination of the pellet or microphere of acid-treated clay should be below about 1500.degree. F. Following the first calcination step, the extraction of the additional alumina from the calcined clay may be carried out by a single acidtreating step with one of the above acids or by a plurality of acidtreating and calcination steps, which follow the first acid-treating step and without altering the form of the particle in a substantial sense. The resultant particle produced by the multiple step of calcination and extraction has had its crystal structure substantially destroyed and if the extraction of the alumina is carried to a sufficient degree, the crystallinity of the clay is substantially completely destroyed as is evident from its X-ray diffraction pattern.
The resultant treatment produces a particle of improved pore volume in that a substantially greater percentage of the pore volume is in pores having radii in excess of about 20 Angstrom units and particularly above 30 Angstrom units than is found in the original acid-treated and initially calcined clay. The particle has a useful catalyst holding capacity, satisfactory hardness, and thermal stability and is a useful catalyst support for many processes since it is substantially chemically inert in processes such as referred to below. It has also a substantially greater heat stability than the calcined particle of lower SiO.sub.2 content.
The resultant particle may be impregnated with a solution of metal salts which are converted into a form suitable for catalytic purposes such as metal or metal compound, e.g., as element, oxide, or sulfide. Since the procedure when employed with conventional catalyst supports is well known, it need not be further described in detail.
This invention may be applied to various clays, such as kaolin or kaolinitic clays, for example, halloysite or ball clay, or the sub-bentonite clays, i.e., the alkaline earth metal montmorillonites.
I prefer, however, to employ for the purposes of my invention the aforesaid sub-bentonite clays.
The product of my invention is useful in catalytic processes, for example, those in which the prior art acid-treated clays with or without catalysis promoting cations have been employed and in many other processes.